The design and development of insulation materials, including such materials for refrigeration structures, is an extensive art. Many systems have been developed employing as the major insulation materials various fibrous and powder products. There have been many ways in which such materials have been utilized, including the use of evacuated shells and bags, compression of the insulating material, and varying orientations of the material. Many of the previously developed insulating materials have proven quite adequate for the purpose as, for example, U.S. Pat. No. 2,768,046, Evans; U.S. Pat. No. 2,867,035, Patterson, Jr.; and U.S. Pat. No. 3,179,549, Strong et al, each assigned to the assignee of the present invention.
The prior art, as indicated, has described the use of powdered insulating materials enclosed within some type of container. For example, U.S. Pat. No. 2,989,156, Brooks et al, describes a heat insulating panel for refrigerators and freezers where a panel is formed from two metal sheets which are sealed, the central portion evacuated and filled with an expanded perlite. The Kummermehr et al patent, U.S. Pat. No. 4,399,175, describes the pressing of finely divided insulation material held within an outer container to form insulating slabs. An insulation system with a rigid outer wall, a flexible inner wall, and powdered insulating material in the space between the walls is described in Schilf, U.S. Pat. No. 4,349,051. A similar structure is shown in Matsch et al, U.S. Pat. No. 3,166,511. Further, hermetically sealed metallic shells filled with powdered materials are described in Munters, U. S. Pat. Nos. 2,067,015 and 2,164,143.
The use of an artificially prepared silica material as an insulating material is described in U.S. Pat. No. 4,159,359, Pelloux-Gervais et al. The artificially prepared silica material of that patent is a fumed silica formed by the heat treatment of a silane material to produce the desired silica particles. Specific parameters for the resulting fumed silica, which is a relatively expensive form of synthetically prepared silica, are set forth in that specification.
An interesting aspect of the Pelloux-Gervais et al patent is the fact that it specifically sets forth that precipitated silica powders have too high a thermal conductivity and, thus, are of no interest as insulating materials.